1. Field of the Invention
The present invention relates to a device for the compensation of chromatic dispersion comprising a compensating optical fibre with a core region surrounded by a cladding region in which a plurality of holes is present which run along the compensating optical fibre longitudinally.
Furthermore, the present invention also relates to an optical communication line comprising a transmission optical fibre length having, at a wavelength λ, a positive chromatic dispersion slope st and/or a positive chromatic dispersion coefficient Dt and a compensating optical fibre length with a negative chromatic dispersion slope sc and/or a negative chromatic dispersion coefficient Dc.
Furthermore, the present invention also relates to an optical communication system comprising such an optical communication line and a method for designing a compensating optical fibre so that, at a predefined wavelength λ, it has a negative chromatic dispersion slope sc and/or a negative chromatic dispersion coefficient Dc.
Throughout the present description and claims, the expression                “chromatic dispersion coefficient D” is used to indicate the first order dependency of the group velocity from the wavelength. In particular, the chromatic dispersion coefficient D is expressed as follows (Govind P. Agrawal, “Nonlinear Fiber Optics—Second Edition”, Academic Press, pages 8-10)   D  =                    ⅆ                  β          1                            ⅆ        λ              =                  -                              2            ⁢            π            ⁢                                                   ⁢            c                                λ            2                              ⁢              β        2            where β1 and β2 are the constant of propagation of the mode of the first and, respectively, of the second order and D is expressed in ps/(nm*Km);        “slope of chromatic dispersion s” is used to indicate the derivative, with respect to the wavelength, of the chromatic dispersion coefficient D and is expressed in ps/(nm2*Km); and        “transmission optical fibre” is used to indicate an optical fibre used in a line or in an optical communication system for the transmission of optical signals from one point to another one located at a significant distance.        
2. Description of the Related Art
As known, the optical fibres are commonly used in the field of optical telecommunications for the transmission of signals. Essentially they comprise an inner cylindrical region, called core, within which a signal is transmitted and an outer annular region, called cladding. The cladding has a refractive index lower than that of the core in order to confine the signal transmitted within the latter.
Typically, the core and the cladding are made from a silica based vitreous material and the difference in the refractive index between the core and the cladding is obtained by incorporating suitable additives (dopants) in the vitreous matrix of the core and/or of the cladding.
In the field of optical telecommunications and the propagation of an optical signal in an optical fibre, the chromatic dispersion (or second order dispersion), defined by the above-mentioned coefficient D, is a phenomenon for which different spectral components of a pulse of light that propagates in an optical fibre travel at different speeds causing a time spreading of the pulse.
In an optical communication system, the chromatic dispersion limits, therefore, the maximum data transmission speed (that is, the bit rate) or the maximum length of a link without electrical regeneration of the signal.
Furthermore, chromatic dispersion is a phenomenon that depends on the wavelength so that pulses of light at different wavelength propagate in an optical fibre at different speeds.
This last phenomenon, known as dispersion of the third order or chromatic dispersion slope (or “slope”) is a problem in wavelength division multiplexing (WDM) optical communication system in which the information is transported along the same optical fibre by a plurality of optical signals at different wavelengths.
In the WDM optical communication systems, therefore, it is necessary to compensate not only the chromatic dispersion but also the chromatic dispersion slope in the range of wavelengths of interest.
Devices to compensate the chromatic dispersion as well as the chromatic dispersion slope of a conventional single mode fibre (SMF) are known.
For example, devices are already known that comprise an optical fibre suitably designed to have values of the chromatic dispersion coefficient D and of the chromatic dispersion slope that are very high and of an opposite sign with regards to those of the SMF optical fibre for which dispersion compensation is required.
Since the SMF optical fibres have a chromatic dispersion coefficient D and a chromatic dispersion slope s that are positive, the devices to compensate the dispersion, typically, have negative chromatic dispersion coefficient and chromatic dispersion slope.
T. Kashiwada et al. (“Broadband dispersion compensating module considering its attenuation spectrum behaviour for WDM system”, OFC '99, WM12, pages 229-231) describe an optical fibre, to compensate the dispersion of a SMF optical fibre with a W refractive index profile designed in order to obtain negative values of the chromatic dispersion coefficient D and of the chromatic dispersion slope so as to compensate the positive values of the SMF fibre.
G. E. Berkey et al. (“Negative slope dispersion compensating fibers”, OFC '99, WM14, pages 235-237) describe an optical fibre, to compensate the dispersion of a SMF optical fibre, with a refractive index profile designed in order to obtain negative values of the chromatic dispersion coefficient D and of the chromatic dispersion slope s in order to compensate the positive values of the SMF fibre.
L. Gruner-Nielsen et al. (“Design and manufacture of dispersion compensating fibre for simultaneous compensation of dispersion and dispersion slope”, OFC '99, Technical Digest WM13, pages 232-234), describe an optical fibre, to compensate the dispersion of a SMF optical fibre, with a depressed cladding and designed to obtain negative values of the chromatic dispersion coefficient D and of the chromatic dispersion slope s in order to compensate the positive values of the SMF fibre.
Furthermore, in the last few years the dispersion properties of a holey optical fibre have been studied.
A holey optical fibre is an optical fibre typically made of a single type of material in which the difference in the refractive index between the core and the cladding, which provides a guided propagation, is achieved through the presence of holes in the cladding which reduce the refractive index of the material from which the fibre is made.
In particular, a holey optical fibre has a cladding region comprising holes that run along the entire length of the fibre and a core region determined by the absence of at least one hole in the material.
The U.S. Pat. No. 5,802,236 describes a micro-structured optical fibre which includes a solid silica core region surrounded by a inner cladding region and an outer cladding region. The cladding region has capillary holes that extend in the axial direction of the fibre. The holes in the outer cladding region are of a smaller diameter than the holes in the inner cladding region and therefore the effective refractive index of the outer cladding region is greater than the effective refractive index of the inner cladding region. This document states that this type of fibre may have high negative values of the chromatic dispersion coefficient D (for example, values that are more negative than −300 ps/nm*Km) at a predetermined wavelength (for example, 1550 nm) and high negative values of the chromatic dispersion slope s so that the fibre can carry out a dispersion compensation in a range of wavelengths of 20 nm or more. Furthermore, a micro-structured optical fibre is described in which the holes of the outer cladding region are of a diameter equal to 0.688 μm, the holes of the inner cladding region have a diameter equal to 0.833 μm and the centre to centre distance Λ between the holes is 0.925 μm.
D. Mogilevtsev et al. (“Group velocity dispersion in photonic crystal fibers”, Optics Letters, Vol. 23, No. 21, November 1998, pages 1662-1664) study the properties of dispersion of photonic crystal optical fibres and indicate that these fibres may have high values of normal (negative) chromatic dispersion coefficient D suitable to compensate the chromatic dispersion at 1550 nm.
S. E. Barkou et al. (“Dispersion properties of photonic bandgap guiding fibers”, OFC '99, FG5, pages 117-119) investigate the dispersion properties of a holey fibre. They point out that, by changing the distance Λ between two centres of two adjacent holes from 1.4 μm to 2.9 μm and in the range of wavelengths between 1.2 and 1.7 μm, very high positive values of the chromatic dispersion coefficient can be achieved. Furthermore, for a value of Λ equal to 2.9 μm they point out that a very flat dispersion curve can be achieved with a dispersion value approximately equal to zero at 1550 nm. Therefore, the Authors conclude that the holey optical fibres have dispersion characteristics very different from the conventional ones, that may be designed for flat, non-zero dispersion over a wide wavelength range and that may exhibit dispersion significantly above the material dispersion.
T. M. Monro et al. (“Holey optical fibers: an efficient modal model”, Journal of Lightwave Technology, vol. 17, No. 6, June 1999, pages 1093-1102) describe a model for the propagation of light in holey optical fibres. With this model the Authors obtain a curve of the chromatic dispersion coefficient as a function of wavelength for Λ being equal to 2.3 and values of the ratio d/Λ (where d is the diameter of the holes) equal to 0.1, 0.2 and 0.3. They indicate that as the hole size is increased, the dispersion value induced by the holes increases (as when the holes are small the chromatic dispersion is dominated by the material dispersion).
T. A. Birks et al. (“Dispersion compensation using single-material fibers”, IEEE Photonics Technology Letters, Vol. 11, No. 6, June 1999, pages 674-676) propose a model for the propagation of light in holey optical fibres according to which they approximate a holey fibre with holes of a large diameter with a fibre made up of a silica core in air. Through the use of this model, the authors calculate that such fibres may have, at 1550 nm, a negative value of the chromatic dispersion coefficient of −2000 ps/(nm*Km) in order to compensate for a conventional optical fibre with a length 100 times greater. Furthermore, they calculate that such fibres may have, at 1550 nm, a chromatic dispersion slope of −2.3 ps*nm−2*Km−1 with a negative chromatic dispersion value of −680 ps/(nm*Km) so that they may compensate a conventional optical fibre with a length 55 times greater over a 100 nm band centred around 1550 nm. Therefore, the Authors conclude that such fibres have great potential for the compensation of chromatic dispersion.
T. M. Monro et al. (“Holey fibers with randomly arranged air holes”, CLEO 2000, San Francisco (USA), 7-12 May 2000, CFJ2 page 670) present a discussion aimed at illustrating that the light may be guided in a holey optical fibre with air holes arranged randomly. They also indicate that in a holey fibre the d/Λ ratio determines a range of possible dispersion values and that when the holes are large they can obtain high coefficient values of chromatic dispersion that may be normal (negative) or anomalous (positive).
A holey optical fibre may be produced by forming a preform starting from a bundle of empty tubes arranged according to a predetermined structure in which the central tube is replaced by a solid bar which will make up the core region of the fibre. The preform achieved in this way is then spinned in order to achieve a holey optical fibre with holes of diameter d and pitch Λ. The spinning process generally allows for the initial structure of the arrangement of the tubes in the preform to remain almost unchanged and, therefore, to achieve the desired value of pitch Λ with a high level of precision.
However, with regards to the diameter d of the holes, the Applicant has noted that during the spinning process of the preform, the diameter of the tubes may vary so that it is not be possible to achieve with as much precision the desired value of the diameter d (and, therefore of the ratio d/Λ).